1. Field of the Invention
Described herein is a fuel cell that includes a reservoir for electrolyte. Specifically, the fuel cell may include one or more “sponge reservoirs” which comprise a portion of the porous substrate which is uncompressed and in fluid communication (and generally integrally formed) with the substrate which is compressed between the electrodes.
2. Description of the Related Art
For the purposes of public safety on the roads and elsewhere, there is a need to make sure that individuals are not operating potentially dangerous machines (such as automobiles) while they are impaired by the effects of alcohol consumption. To try and prevent people from driving drunk, most states have enacted laws that impose fines or other criminal penalties if individuals have a breath or blood alcohol level above a certain amount. In order to effectively enforce these laws, it is necessary to be able to measure the alcohol concentration in human breath and compare the results against legal limits. There are a variety of measuring instruments used for determining the concentration of alcohol in human breath ranging from small hand held devices to larger bench top units and machines built into cars or certain machinery. Since a determination of breath alcohol above the legal threshold can result in criminal penalties, loss of a job, or other sanctions, the accuracy of these instruments is paramount.
Fuel cells which are being used as sensors, and particularly fuel cells as alcohol sensors such as may be used in breath alcohol sensors, have typically had the active element constructed as shown in FIG. 1. This assembly (100) is typically manufactured by arranging the components and then compressing the assembly (100) under pressure in order to get the material of the electrodes (101), generally platinum, to adhere to the substrate (103). In the process, the porous substrate (103) is typically permanently deformed resulting in a smaller effective pore size of the substrate compared to its pore size prior to pressing. It is these pores of the substrate (103) that hold liquid electrolyte to allow the fuel cell (100) to function and the pores are typically filled by soaking the assembly (100) after pressing. Once filled with liquid electrolyte, the active element (100) can act as a fuel cell.
Typical electrode assemblies might be round or square as shown in FIG. 2 and are typically less than 1 mm thick. Substrates (103) are generally a porous material such as, but not limited to, porous PVC and porous polypropylene, but there are many candidate materials. Those skilled in the art will understand that these assemblies (100) are, therefore, generally very small and therefore typically will hold a very small amount of electrolyte. Five one-hundredths (0.05) of a milliliter would not be an unusual quantity.
Fuel cell sensors in portable equipment, such as those used by highway patrol officers, by definition, must operate in a variety of ambient conditions such as hot, cold, humid, and arid environments. Hot and/or arid conditions tend to draw water out of the fuel cell electrolyte as the water in the electrolyte tries to reach equilibrium with the environment. Even if the electrode assembly (100) is enclosed in a case (300), water can be drawn out through the case material, which is rarely completely air and fluid sealed, or through necessary sampling ports (301) allowing the fuel cell (100) to be used in the breath alcohol sensor as is shown in FIG. 3. Previously attempted remedies against water loss generally add cost and complexity and are not 100% effective, especially over an expected sensor life in the marketplace measured in years, and especially in a punishing environment.
In some cases, fuel cells (100) are used in indoor bench top equipment. Although this is typically a moderated environment compared to the great outdoors, indoor conditions can still be quite arid, especially in winter. Fuel cells (100) are also often heated in bench top equipment for measuring reasons, generally making the fuel cell microenvironment even drier than the overall room conditions.
Those skilled in the art will understand that if all the water, or nearly all the water, is drawn out of a fuel cell (100) electrolyte, the fuel cell (100) will cease to work and may become permanently damaged. Various degrees of water loss short of 100%, or nearly 100%, typically do not keep a fuel cell (100) from measuring accurately. However, the response time may become slower, for example.
Over the years, a variety of methods have been used to deal with electrolyte water loss. In one example where the gas sample is human breath (which is common in a breath alcohol detector), every time a sample is taken, moisture will be added to the substrate (103) from humidity in the breath as shown in FIG. 4. However, the amount of moisture added per sample is generally so small, and the typical number of tests run on an instrument over a year is also small enough that these additive effects are typically swamped by the opposite effect of water loss.
Humid ambient conditions (which result in less water loss and possibly even water gain through the inverse of the above processes) are more likely to be the driving force behind water gain in a fuel cell than breath addition. A fuel cell sensor (100) containing an electrolyte can take on water from very humid ambient conditions. The paths of water gain from the ambient are the same as water loss to the ambient, only in reverse. Those skilled in the art will understand that, while this can be beneficial, if the sensor substrate (103) is already saturated with liquid, it may continue to take on liquid from the humid ambient conditions until the volume of such liquid exceeds the designed containment capacity of the sensor (100). In this case, liquid can overflow the sensor (100) and appear on the electrode (101) surfaces or other locations where it will likely hinder the intended operation of the sensor (100).
Certain fuel cell designs have allowed manufacturers to experiment with manually adding drops of water directly to an exposed electrode (101) (anode) of the fuel cell (100) when it has dried out significantly as shown in FIG. 5. Results have been mixed as water loss often causes the cell (100) to reach a tipping point where adding water back does not reverse the effects of losing it in the first place. Further, at times the drier platinum electrode (101) becomes partially hydrophobic and the added liquid can take considerable time to soak in, if it can at all. Thus, addition via the anode side of the fuel cell (100) is often ineffective.
It is generally believed that the ideal electrolyte situation is to constantly keep the substrate (103) filled from the very beginning of the fuel cell's (100) useful life. For many years, some fuel cell sensor (100) manufacturers have been adding “backup” disks (107) in the construction of fuel cell sensor (100) assemblies. This is an extra disk of substrate without an attached electrode that is typically assembled behind the cathode (101). The backup disk (107) has typically been made of the same substrate material as the electrode disk (103), but without any compression, leaving the pores in their original state. Therefore, this disk (107) will hold more electrolyte than the pressed version.
The backup disk (107) acts as a reservoir for extra electrolyte to replenish the electrolyte in the substrate (103) between the electrodes (101) when water is lost to ambient conditions. The smaller (compressed) pores of the substrate (103) between the electrodes (101) preferentially stays full compared to the backup disk (107) due to capillary action since the backup disk (107) pores are larger. FIG. 6 provides an embodiment of such a backup disk (107).
This solution has worked reasonably well in the field in many instances. There are a couple of drawbacks with this construction, however. The complete fuel cell sensor assembly (100) generally includes wires (111) to connect the electrode (101) surfaces to an external circuit (109) for measuring the current produced from a gas sample. This construction typically places a wire (111) between one electrode surface (101) and the backup disk (107) which to some degree prevents a perfect contact surface between electrode (101) and backup disk (107) as shown in FIG. 7. Thus, the capillary action is somewhat hindered between the backup disk (107) and the substrate (103). Also, as mentioned above, the electrode (101) surface itself presents an additional layer through which the moisture must travel and may detract from an ideal capillary action.
As a final note, there is no good way to tell whether a fuel cell (100) is flush with electrolyte or starved for electrolyte unless or until a large degradation in performance becomes apparent. By the time this happens, it is often too late to reverse that degradation by adding water or electrolyte and the fuel cell (100) is effectively destroyed.